Breakthrough for crack propagation in glass

Researchers have long pondered the origin of delicate crisscross faceted patterns that are commonly found on the surfaces of broken material. Typical crack speeds in glass easily surpass a kilometre per second, and broken surface features may be smaller than a millimetre. Since the formation of surface structure lasts a tiny fraction of a second, the processes generating these patterns have been largely a mystery.

However, replacing hard glass with soft but brittle gels makes it possible to slow down the cracks that precipitate fracture to mere metres per second. Using this technique has enabled researchers at the Hebrew University of Jerusalem’s Racah Institute of Physics to unravel the complex physical processes that take place during fracture in microscopic detail and in real-time.

The work sheds light on how broken surface patterns are formed. Surface facets bounded by steps are formed due to a special ‘topological’ arrangement of the crack that cannot easily be undone, much as a knot along a string cannot be unravelled without pulling the whole length of the string through it.

These ‘crack knots’ increase the surface formed by a crack, thereby creating a new venue for dissipating the energy required for material failure, and thereby making materials harder to break.

“The complex surfaces that are commonly formed on any fractured object have never been entirely understood,” said lead researcher, Professor Jay Fineberg. “While a crack could form perfectly flat, mirror-like fracture surfaces, generally complex faceted surfaces are the rule, even though they require much more energy to form. This study illuminates both how such beautiful and intricate patterns emerge in the fracture process, and why the crack cannot divest itself of them once they are formed.”